Integrated circuits (ICs) are fabricated on wafers. Commonly, these wafers are semiconductor materials, such as silicon, and, singulated to form individual dies. Through efforts of research and development, the size of the transistors making up the ICs has decreased to 45 nm and will soon decrease to 32 nm. As transistor size decreases, the supply voltage to the transistors decreases. The supply voltage is conventionally smaller than wall voltages available in most countries or battery voltages used in portable devices. For example, an IC may operate at 1.25 Volts whereas the wall voltage is 120V or 240V. In a portable device, such as cellular phone, the battery voltage may range from 6V at full charge to 3V at near empty charge.
A semiconductor die may be coupled to a voltage regulator that converts available voltages at wall outlets or batteries to lower voltages used by the die. The voltage regulator ensures a constant voltage supply is provided to the die. This is an important function, because the ability of transistors to tolerate voltages under or over the target voltage is small. Only tenths of a volt lower may create erratic results in the die; only tenths of a volt higher may damage the die.
Dies are mounted on a packaging substrate, and the packaging substrate is mounted on a printed circuit board (PCB) approximately 1-2 mm thick during assembly. Conventionally, the voltage regulator is located on the PCB with the die to which the voltage regulator supplies voltage. Placing the voltaize regulator on the PCB separate from the die results in a voltage drop between the voltage regulator and the die that the voltage regulator supplies. For example, at a supply voltage of 1.125 Volts, a voltage drop of 0.100V may occur between the voltage regulator and the die as the voltage passes through the PCB, packaging substrate, and die. As the supply voltage decreases with shrinking transistor size, the voltage drop becomes a significant fraction of the supply voltage. Additionally, placing the voltage regulator on the PCB requires the use of pins on the die to allow the die to communicate with the voltage regulator. The die may send commands to the voltage regulator such as sleep or wake-up for scaling up or scaling down the voltage supply. The additional pins consume space on the die that could otherwise be eliminated.
Reducing the voltage drop from the voltage regulator to the die improves performance of the die. Maximum frequency of a die scales proportionally with supply voltage. For example, eliminating a voltage drop of 0.100V may increase a maximum frequency (fmax) of the die by 100 MHz. Alternatively, if the voltage drop is reduced and maximum frequency not increased, power consumption in the die is reduced. Power consumption is proportional to capacitance multiplied by a square of the supply voltage. Thus, reducing the supply voltage may result in significant power savings.
Further, conventional voltage regulators have slow response times due to the distance between the voltage regulator and the die. In the event the current transients are too fast for the voltage regulator to respond, decoupling capacitors provide additional power to the die. Voltage regulators located on the PCB often have response times in the microsecond range. Thus, large decoupling capacitors are placed on the packaging substrate to compensate for slow response times. The large decoupling capacitors occupy a large area. One conventional arrangement includes a bulk capacitor of microFarads and a multi-layer chip capacitor (MLCC) having hundreds of nanoFarads along with the voltage regulator on the PCB. The combination of the bulk capacitor and the MLCC supplies voltage to the die while the voltage regulator responds to the current transient.
Attempts have been made to place voltage regulators on the dies. However, voltage regulators include passive components such as inductors and capacitors that are also embedded in the dies. Passive devices consume die area, which increases manufacturing cost. For example, a die manufactured using 45 nm technology has a capacitance density of 10 femtoFarads/μm2. At this density a suitable amount of capacitance may consume over 2.5 mm2. Providing inductance to the voltage regulator conventionally uses an on-die inductor or a discrete inductor mounted on the packaging substrate. In addition to consuming large areas on a die, conventional on-die inductors have a low quality factor.
A quality factor for passive components embedded in a die is low because the passive components are manufactured thin to fit in the die. As the amount of conducting material shrinks, conductive or magnetic losses increase and degrade the quality factor. The quality factor is defined by the energy stored in a passive component versus energy dissipated in the passive component, for a passive component embedded in a die is low.
Thus, there is a need for a voltage regulator that is in close proximity to the die without consuming large amounts of die area.